Various types of respiratory devices are well known in the art, and the present invention deals with that class of devices generally referred to as either resuscitators and/or ventilators, depending upon the primary intended usage. One prior art device is a "Bear II Ventilator". In one mode of operation the ventilator forces a tidal volume of an air oxygen mixture into a patient's lungs for a prescribed period of time, for example two seconds, and then permits the patient's respiratory cavity to collapse thereby expelling the air oxygen mixture for another prescribed period of time, for example four seconds. This same ventilator may also be operated in another mode wherein, in addition to operating in a timed cycle, it would also sense through pressure changes spontaneous inspiratory effort by the patient, which sensed changes would cause the ventilator to switch from expiratory mode to an inspiratory mode. Thus, if the ventilator were in the expiratory mode, and the patient desired to inhale, electronic circuitry could cause the device to shift into its inspiratory mode.
While devices similar to the "Bear" unit are for hospital use, it is desirable that a lightweight portable ventilator/resuscitator be developed which is capable of utilizing filtered ambient gases, to supplement an internal source of pressurized breathing gases, the ambient gases passing through a filter to remove toxic or harmful contaminants. Such a device would find utility with the Armed Forces, for example where servicemen may be subjected to nerve gas attacks. Such a portable ventilator/resuscitator should initially operate from an internal power supply, such as the source of pressurized breathing gases (which may be an oxygen generator or a container of compressed gas), but it is desirable that upon failure of the internal power supply that it can also be operated in a manual mode.